Generic tow lead-in for streamers

ABSTRACT

Generic tow lead-in for streamers providing communication between the seismic systems and the streamers, consisting of at least four wire power quad, at least four multimode optical fibers and at least one signal pair, where the at least one signal line do not utilize a screen.

This is a 371 filing of International Patent Application No.PCT/NO2005/000290 filed Aug. 15, 2005 and published on Feb. 22, 2007under publication number WO 2007/021192 A.

The present invention is related to a generic tow lead-in for streamersaccording to the preamble of the claims.

3D seismic surveys are today carried out by vessels towing multiplestreamers, e.g. cables that contain a number of hydrophone groups alongthe length of the streamers. The lengths deployed are in the range offrom 600 up to a maximum today of 12 000 meters per cable. The number ofcables deployed at any one time can be anything from 2 to 20 and thisfigure is increasing.

The lead-in cables provide a link between the streamer winch and theactual streamer. Such cables are typically triple layer armoured and aredesigned to withstand the harsh environment, encountered at the forwardend of the streamer towed array. There are also short lengths ofstreamer of 100 meters each at the forward and aft end of the streamer.These sections have a stretch facility and absorb the axial tuggingencountered at sea. Lead-ins are armoured cables, used to link thestreamer to the vessel. They do not carry hydrophones, but comprisepower supply, auxiliary conductors and fibre optic data transmissionlines.

A paravane is a device that when tethered to a fixed object, and has amoving fluid across its surface, produces a lift force, and a dragforce. A fixed bridle tows the paravanes, and the angle to the flow canbe varied to achieve the required adjustment of lift force. They arenormally towed just below the surface of the water, and have stabilityas a result of centre of buoyancy being above the centre of gravity.

Paravanes are used to provide separation at the head of the streamers,the head being the vessel end of the towed array. These forwardparavanes may have lift figures close to 15 tonnes and can have asurface area of up to 40 square meters.

Typical survey speed on vessels towing multiple streamers is today 4-5knots. The speed is limited by the transverse forces acting on the towlead-in, in particular the outermost ones, as they have the steepestangle relative to the vessel direction. Furthermore, it happens that thebridle for the paravanes break and then the forces taken up by thebridle is transferred to the tow lead-in cables. The above mentionedforces have resulted in a very robust design of the tow lead-in cables,to prevent them from snapping due to a sudden increase in tension due tosnapping of the bridle for at least one of the paravanes, mainly basedon trial and error. Thus leading to cables with bigger/greater diameterswhich again limit the speed of the vessel.

The above mentioned disadvantages with prior art tow lead-in cables areavoided with the tow lead-in according to the present invention asdefined by the features stated in the claims.

The drawing discloses in

FIG. 1 schematically a prior art streamer deploy showing only streamerson one side behind a vessel,

FIG. 2 discloses a cross section of the tow lead-in cable according topresent invention and

FIG. 3 show a typical optic-electric core for use in tow lead-in cablesfor the seismic exploration and defence market.

Most seismic surveys are carried out in the way shown in FIG. 1. Avessel 5 is towing one or several streamers 3 with lengths between 600and 12000 meters per cable. Tethers 16 are used at the front end of thestreamers to maintain a constant distance between the streamers. Theparavane 4 pull the streamers transversely relative to the direction ofthe vessel so as to increase the amount of cables that can be deployedin an seismic survey. To guide the paravane 4, a bridle 1 is connectedbetween the paravane 4 and the vessel 5, and is designed to withstandhigh tensile forces. The tow lead-in 2 is connected between thestreamers 3 and the vessel 5 and provide the streamers 3 with power andthe means to communicate with the seismic equipment onboard the vessel.

The leading system providers on the market, like MSX, Syntrak and Seal,put forward certain requirements as to the power-, signal- anddata-lines so that all the products on the streamer are able to functionproperly. These requirements are minimum resistance for the power line;attenuation, frequency and transmission length for the signal line; andfour multimode optical fibres for the data lines.

To be able to reduce the size/diameter of the tow lead-in, one has tolook at the core to see if the parts concerning these systemrequirements can be altered and the need for reinforcement for strengthto withstand the maximum tension acting on it. Given the power line 13and optical fibre components 12, cable size reduction can only beachieved by reducing the signal component sizes 11,15 and increasing thespace effectiveness of the cable geometry.

The losses in a signal line 11,15 are composed of three elements, theconductor wire losses (loop resistance), the dielectric losses(insulation type), and the screen losses. By eliminating the signal linescreen a higher resistance wire pair can achieve the same attenuation aspreviously.

The prime reason for screening 17 signal lines, is the reduction ofnoise from other adjacent lines. This is commonly referred to ascross-talk. Cross-talk is the logarithmic ratio of the power induced onone line with respect to the power transmitted on another line. It isgiven the units bels, or more frequently decibels (one tenth of a bel).

The electrical-optical core is geometrically constructed such that thecable is symmetrical and minimises differential cross-talk. The cable isalso constructed in a uni-lay form to minimise diameter. A typicalelectrical-optical core is shown in FIG. 3. To ensure structuralstability under radial compression, the signal quads 11 and pairs 15 areindividually belt fill extruded to a round shape. To ensure good fatiguelife the signal conductors are manufactured from high tensilecopper-cadmium alloy. The core is further reinforced for strength withat least one layer for strength which can be of high tensile steel,aramid fibre, PBO fibre, or HMWPE fibre. An outer sheath may be appliedfor protection, though this is optional for steel armoured cables.

Under testing it was found that the crosstalk between the central powerquad and signal pairs is better than 90 dB for frequencies up to 5 kHz.The crosstalk between quads and pairs is better than 60 dB forfrequencies up to 500 kHz. These results are measured over a 1000 mcable. Testing of cables with common/individual screen give similarresults. The key difference between cables with and without screen isthe size. The size and shape of the cable has consequence on the dragforce and the vortex induced vibrations when towed in water with anangle relative to the direction of motion.

To meet the attenuation and crosstalk levels seen with a screened orshielded unit would result in a component nearly twice as large. The newcore cable has a diameter some 70% of what would be expected and across-sectional area some 50% of what would be expected.

The cable is suitable for systems using power lines from DC up to 5 kHz,and where up to 12 signal lines 11,15 are required. The constructionalso contains 4 optical fibres 12, though this could be extended to 8optical fibres when two fibres per tube are used.

FIG. 2 shows one embodiment according to this invention where the corefrom FIG. 3 is used as basis to optimise the diameter of the cable. Inthis embodiment the core is further reinforced for strength with twolayers of high tensile steel and an outer sheath have been applied forprotection.

The cable according to the invention can be used in any applicationrequiring transmission of signal, data, and power where diameter andperformance are at a premium.

1. Generic tow lead-in for streamers providing communication betweenseismic systems and the streamers, consisting of at least four wirepower quad, at least four multimode optical fibres and at least onesignal pair, wherein the at least one signal pair does not utilize ascreen and has a resistance of the signal pair increased to compensatefor an attenuation change from a signal pair utilizing a screen. 2.Generic tow lead-in according to claim 1, wherein the core is coveredwith at least one reinforcement layer for strength.
 3. Generic towlead-in according to claim 2, wherein the at least one reinforcementlayer is high tensile steel.
 4. Generic tow lead-in according to claim2, wherein the at least one reinforcement layer is aramid fibre. 5.Generic tow lead-in according to claim 2, wherein the at least onereinforcement layer is PBO fibre.
 6. Generic tow lead-in according toclaim 2, wherein the reinforcement layer is HMWPE fibre.
 7. Generic towlead-in according to claim 1, wherein the crosstalk between the centralpower quad and signal pairs is better than 90 dB for frequencies up to 5kHz.
 8. Generic tow lead-in according to claim 1, wherein the crosstalkbetween the quad and signal pairs is better than 60 dB for frequenciesup to 500 kHz.
 9. Generic tow lead-in according to claim 1, wherein thecable can be used for systems using power lines from DC up to 5 kHz. 10.Generic tow lead-in for streamers providing communication betweenseismic systems and the streamers, comprising; at least four wire powerquad; at least four multimode optical fibres; and a plurality of signalpairs, at least one signal pair having a screen, wherein at least onesignal pair does not have a screen and wherein a diameter of the towlead-in is reduced by the at least one signal pair not having a screen.11. Generic tow lead-in for streamers providing communication betweenthe seismic systems and the streamers, the lead-in having powerconductors, a plurality of multimode optical fibers, and a plurality ofsignal pairs with screens wherein the improvement comprises a diameterof the tow lead-in being reduced by at least one of the signal pairs nothaving a screen.